DESCRIPTION: Mordenite is a naturally occurring mineral found in many locations throughout the United States and the world. Recently, it was discovered in the subsurface of the proposed nuclear waste repository in Yucca Mountain, Nevada where workers were reportedly being exposed to fibrous mordenite aerosols during drilling operations. Several animal and human studies have reported results suggesting that mordenite is a mesothelial carcinogen. Further concern that the fibrous nature of mordenite presents an inhalation hazard is supported by the "Stanton Hypothesis" stating that fiber carcinogenicity is related to its morphology and biopersistence rather than its physicochemical properties. Previous research has shown that aerosolized mordenite fibers can achieve morphological properties consistent with the "Stanton Hypothesis," but their biopersistence in the pulmonary environment is poorly understood. Previous studies have evaluated the in-vivo biopersistence of other fiber types by measuring fiber dissolution in simulated lung fluid. This study will use a similar experimental design and evaluate the in vivo biopersistence of mordenite by measuring fiber dissolution in an acidic simulated lung fluid. The research hypothesis to be tested is: no difference in chemical or dimensional fiber characteristics exists before and after in-vitro dissolution using simulated lung fluid at acidic pH (4.5-5.0). The specific aims to test the hypothesis are: generation and collection of aerosolized mordenite fiber samples, chemical and physical characterization of mordenite fiber samples, analysis of dissolution effluent for mordenite fiber dissolving elements, calculation of dissolution rates for the mordenite network mineral (silica) and any leached elements. Performance of this research will allow health professionals to make a more appropriate hazard risk determination concerning mordenite and other similar airborne materials thereby offering better recommendations to reduce pulmonary disease.